Java Multiplication with several bases

Question

Hi everyone I stuck on this homework the task is to multiply two BigInteger Numbers. I should use the base k. But the result is not right. Can someone give me a hint i think the problem is the shiftleft.

    public BigInteger multBaseKBit(BigInteger a, BigInteger b, int k) {
    BigInteger result = new BigInteger("0");
    BigInteger tmp = new BigInteger("0");
    for (int i = 0; i < b.bitLength();) {

        if (b.testBit(i)) {
            tmp = a;
            tmp = tmp.shiftLeft(i);
            result = result.add(tmp);
            System.out.println(result);

        }
        i = i + k;
    }
    return result;
}